1. Field of the Invention
This invention relates to light-emitting device drive circuits and optical transmission systems using the circuits and, more specifically, to the circuits for driving a light-emitting device used for an optical transmission circuit and other circuits in an optical communication apparatus, and an optical transmission system using the light-emitting device drive circuit.
2. Description of the Background Art
As being well known, with recent advancement of technology, optical fibers have been able to achieve wide-band, low-loss transmission. Therefore, the optical fibers have come to be applied more to a backbone system for high-speed, large-capacity transmission carried out typically over the Internet. In the future, the optical fibers are expected to be applied further to a backbone-to-home access system, home network, and other networks.
To achieve such next-generation digital home networks, an interface, which is able to transmit a large amount of digital signals at high speed for a long distance with high quality and at low cost, is needed. Among potential protocols of such interface is IEEE 1394, which standardizes digital signals of 100 Mbps, 200 Mbps, 400 Mbps, and other transmission rates. Under IEEE 1394, however, a transmission medium for use is implemented by an electrical cable, which enables transmission only for a short distance of 4.5 m. To make the distance far longer, the transmission should be optically achieved by using an optical fiber, which is not affected by disturbance due to electromagnetic waves, instead of using the electrical cable.
The optical fibers are exemplarily classified into glass optical fibers (hereinafter, GOFs), polymer-clad fibers (hereinafter, PCFs), and plastic optical fibers (hereinafter, POFs), according to the difference of materials. The GOFs are suitable for long-distance transmission over the backbone system, for example, because of their extremely small transmission loss. However, the GOF""s core through which an optical wave passes is so small in diameter (10 to 50 xcexcm) that connectors and other components used in the system have to be made with high accuracy, thereby increasing their manufacturing cost. Moreover, the GOF""s core is made of glass, which is inflexible and easy to be broken, and therefore extreme caution is required in handling the GOFs. The PCF""s core is no less than 200 xcexcm in diameter, which is larger than that of the GOFs, but also made of glass as the GOF""s core. Therefore, extreme caution is required also in handling the PCFs. On the other hand, the POF""s core is approximately 1 mm in diameter, which is extremely larger than those of the other two, and therefore connectors and other components used in the system can be made without requiring high accuracy, thereby reducing their manufacturing cost. Moreover, the POFs are entirely made of plastic material, and therefore they are easy to handle and pose no danger for use at home. Therefore, a POF optical transmission technique based on IEEE 1394 is coming to more attention for realizing an interface of the next-generation digital home network.
The POF""s core is generally made of polymethyl methacrylate (hereinafter referred to as PMMA) type material. FIG. 4 shows transmission loss characteristics of a PMMA-type POF with respect to a wavelength xcex. As shown in FIG. 4, low transmission loss is observed in optical waves with their wavelength bands ranging from 450 to 540 nm, from 560 to 580 nm, and from 640 to 660 nm. Therefore, for high-speed and long-distance signal transmission, a light source suitable for one of those wavelength bands should be selected. For example, a light source for a wavelength band of 640 to 660 nm is selected. Furthermore, in consideration of cost and eye safety when a user directly views light, the light source for use at home or other purposes is preferably a light-emitting diode (hereinafter, LED) rather than a semiconductor laser diode (LD). For this reason, one potential interface is realized by an optical transmission system using the POF and the LED for the wavelength band within 640 to 660 nm.
However, if the LED is selected as the light source, what is concerned is how fast the response is. More specifically, the LED for 640 to 660 nm has a frequency bandwidth of approximately 100 MHz, and therefore digital signals of 200 or 400 Mbps under IEEE 1394 cannot be transmitted through this LED. Therefore, a method of compensating the LED""s bandwidth has been suggested using an electrical circuit.
One example of a conventional light-emitting device drive circuit used in an optical transmission circuit is disclosed in Japanese Patent Laid-Open Publication No. 9-83442 (1997-83442). FIG. 5 is a schematic diagram showing the structure of the conventional light-emitting device shown in this publication. FIG. 6 shows an example of a signal waveform at each component of the conventional light-emitting device drive circuit of FIG. 5. In FIG. 5, the conventional light-emitting device drive circuit includes a signal current source 41 for outputting a signal current i1 corresponding to a transmission signal, a differential current source 42 for outputting a differential current i2 corresponding thereto, a signal adder 43, and a light-emitting device 44.
In general, if the light-emitting device 44 whose bandwidth is insufficient for the transmission signal is driven only by the signal current i1 having a rectangular waveform shown in (a) of FIG. 6, a light output Pout outputted from the light-emitting device 44 has such a waveform as that of the transmission signal with blunt rising and falling edges ((e) of FIG. 6). Such waveform is caused due to the capacity and internal resistance of the light-emitting device 44 itself. With such structure, transmitting an optical signal at high speed can not be achieved. On the other hand, the differential current i2 has a differential waveform with its steep peaks appearing at the rising and falling edges of the transmission signal ((b) of FIG. 6). Therefore, the signal current i1 having a rectangular waveform and the differential current i2 having the differential waveform are added together by the signal adder 43, and an output therefrom is an injection current Iin having a waveform with its steep peaks appearing at the rising and falling edges of the transmission signal. This injection current Iin drives the light-emitting device 44, and an output therefrom is the light output Pout having a desired band-compensated waveform (rectangular waveform) ((d) of FIG. 6).
As being evident from (d) of FIG. 6 showing the waveform of the light output Pout, the light-emitting device 44 constantly emits light even though the digital signal is at the low level (L) (refer to a slanted part in the drawing). Such light emission is unavoidable because predetermined direct current components have to be included in the injection current Iin for preventing waveform distortion that occurs when the peak value at the falling edge of the injection current Iin becomes below zero. If not prevented, this distortion leads to distortion in waveform of the light outputted from the light-emitting device 44.
Such light emission, however, acts as noise, affecting the transmission characteristics to deteriorate a signal-to-noise (S/N) ratio of the digital signal after transmission.
Therefore, an object of the present invention is to provide a light-emitting device drive circuit and an optical transmission system using the circuit that ensure good transmission capabilities by compensating the bandwidth and also bringing the low level of the light output waveform down to zero (or approximately zero) to improve the S/N ratio of the digital signal.
The present invention has the following features to attain the object above.
A first aspect of the present invention is directed to a circuit that drives a light-emitting device based on an inputted digital signal, the circuit comprising:
an npn transistor, supplied at a base thereof with an electrical signal corresponding to the digital signal, for outputting a voltage at a high or low level to an emitter thereof based on the electrical signal;
a current compensation circuit, connected to the emitter of the transistor, for compensating an output current of a collector of the transistor at a rising edge when an emitter voltage of the transistor is at the high level, and at a falling edge when the emitter voltage of the transistor is at the low level;
a signal current source, connected to the collector of the transistor, for amplifying the output current by a predetermined multiplication factor, and outputting a signal current;
a constant-current source, connected to the signal current source in serial, for diverting a part of direct current components of the signal current outputted from the signal current source; and
a light-emitting device, connected to the signal current source in serial and to the constant-current source in parallel and supplied with an injection current obtained by subtracting the diverted part from the signal current, for emitting light based on the injection current.
As stated above, in the first aspect, the current compensation circuit carries out bandwidth compensation, and the injection current obtained by subtracting a part from the signal current drives the light-emitting device. Thus, light emission when the digital signal is low can be sufficiently suppressed at the time of bandwidth compensation of the light-emitting device. Accordingly, deterioration in S/N ratio can be suppressed, thereby achieving high-quality, long-distance transmission.
Here, the current compensation circuit is preferably structured by a first resistor with one end thereof connected to the emitter of the transistor, a second resistor with one end thereof connected to the other end of the first resistor and with the other end grounded, and a capacitor connected to the first resistor in parallel.
Further, the signal current source is a current mirror amplifier composed of at least two pnp transistors.
Still further, the light-emitting device is supplied with the injection current at an anode thereof and is grounded at a cathode thereof.
Still further, the light-emitting device emits light with a wavelength having an optical spectral characteristic of a band from 450 to 540 nm, 560 to 580 nm, or 640 to 660 nm.
Thus, light emission when the digital signal is low can be sufficiently suppressed and, accordingly, deterioration in S/N ratio can be suppressed. Therefore, long-distance transmission can be achieved with high quality.
A second aspect of the present invention is directed to an optical transmission system for transmitting, through a transmission medium, an optical signal outputted from a light-emitting device drive circuit that drives a light-emitting device based on an inputted digital signal,
the light-emitting device drive circuit includes
an npn transistor, supplied at a base thereof with an electrical signal corresponding to the digital signal, for outputting a voltage at a high or low level to an emitter thereof based on the electrical signal;
a current compensation circuit, connected to the emitter of the transistor, for compensating an output current of a collector of the transistor at a rising edge at when an emitter voltage of the transistor is at the high level, and at a falling edge when the emitter voltage of the transistor is at the low level;
a signal current source, connected to the collector of the transistor, for amplifying the output current by a predetermined multiplication factor, and outputting a signal current;
a constant-current source, connected to the signal current source in serial, for diverting a part of direct current components of the signal current outputted from the signal current source; and
a light-emitting device, connected to the signal current source in serial and to the constant-current source in parallel and supplied with an injection current obtained by subtracting the diverted part from the signal current, for emitting light based on the injection current, wherein
to the transmission medium, at least part of an optical signal outputted from the light-emitting device is coupled, and the transmission medium is capable of transmitting at least part of the coupled optical signal.
Here, the current compensation circuit is preferably structured by a first resistor with one end thereof connected to the emitter of the transistor, a second resistor with one end thereof connected to the other end of the first resistor and with the other end thereof grounded, and a capacitor connected to the first resistor in parallel.
Further, the signal current source is a current mirror amplifier structured by at least two pnp transistors.
Still further, the light-emitting device is supplied with the injection current at an anode thereof and is grounded at a cathode thereof.
Still further, the light-emitting device emits light with a wavelength having an optical spectral characteristic of a band from 450 to 540 nm, 560 to 580 nm, or 640 to 660 nm.
As such, in the second aspect, provided is the transmission medium to which at least part of the optical signal outputted from the light-emitting device of the light-emitting device drive circuit in the first aspect is coupled, and the transmission medium transmits at least part of the coupled optical signal. Thus, the optical transmission system according to the second aspect can realize high-quality, long-distance transmission by suppressing deterioration in S/N ratio of the digital signal after transmission.
For the transmission medium, a PMMA-type POF is preferably used. Thus, high-quality, long-distance transmission can be achieved more effectively with the POF.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.